This basically means: try until you run out of time (50 minutes on
Travis). On Linux, we really want the tests to pass, so there is no point
in limiting the number of retries. On windows, we don't retry, on FreeBSD
it's limited to 1.
Having -Werror set by default causes users' builds to fail because
toxcore is not warning-free. Failing on errors is appropriate for the
development phase, e.g. when building it in a CI enviroment, but it
doesn't make much sense to fail builds for users and let them figure out
that they need to pass -DWARNINGS=OFF to make the library build.
